I bought a used 99 Toyota Tacoma V6 3.4L a couple weeks ago (136,000 miles). Two days after I bought it the CEL came on and O'Reilly ran the P0171 code (bank 1 lean 1). In my research on the Tacoma I read that O2 sensors were commonly replaced at this age, so I assumed this was the problem. I brought it to the shop and they cleaned the Mass Air Flow sensor, cleaned the K&N air filter, changed the fuel filter (didn't need replacing, it was clean), and checked for vacuum leaks, which they could not find any. The light was off for all of 10 miles and then came back on. Brought it back and they replaced the O2 sensor, checked again for vacuum leaks and called it good. I left for a trip to California and the light came back on in about 10 more miles. I continued to drive the truck with the CEL on for another 1300 miles with no problems (other than it ran a little rougher than before when accelerating). The fuel economy wasn't bad either, 23 mpg at 65 on hilly Nevada highways. I brought it back to the same shop and this time they replaced the Mass Air Flow sensor (free labor charge, luckily) and test drove it for 40 miles with no light. Yesterday the CEL came on again. I haven't ran the code but I am almost sure it will be P0171 again. Any more ideas on what else could be the problem?

 

Ran the code at O'Reilly and it's now P0420 (Catalyst below Threshold). Read on Tacoma World forums that high ethanol % gasoline could be the problem so I'm going to drive the tank low and put in high octane gasoline. Probably seafoam it too. Hopefully not the catalytic converter. Any experiences with this code?

 

I think that a rear O2 sensor might be worth looking at for that code... I've seen a few posts about it.

 

What brand O2 Sensor was used? These rigs do not like Bosch....use Denso only.